Gastric parietal cell ultrastructual correlation with acid secretory activity is being extended. High secretory rates are induced by combined stimulatory injections of secretagogues. During the near complete apparent transposition of tubulovesicular membranes from the cytoplasm to the microvilli by an undefined mechanism, multivesicular bodies are consistent features of these cells. The possible role that these organelles may play in the transfer of membrane components from one cell compartment to another during both the formation and depletion stages are being investigated. Tracer methods such as horseradish peroxidase, lanthanum will be used to observe the apparent formation of multivesicular bodies from remnants of the intracellular canaliculus. Studies by freeze fracture replica techniques will be used to follow the membrane characteristics during the transitional stages. Studies on the enteric surface coat will be focused on its structure in the unfixed non cryoprotected state by freeze etch methods.